Photographic process



23,952,538 Patented Sept. 13, 1960 2,952,538 PHOTOGRAPHIC PROCESS Hendrik Jonker, Roelof Jan Hendrik Alink, and Tijs Willem van Rijssel, Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Co pany, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Feb. 15, 1-956, Ser. No. 565,532 Claims priority, application Netherlands July 25, 1950 11 Claims. ,(CI. 96-48) This invention relates to a process and material for producing photographic contrasts with the aid of light.

This application is a continuation-in-part of our application .'Serial No. 236,068, filed July 10, 1951, now U.S. Pat ent 2,73 8,272. Photographic contrasts have been produced by exposure of a carrier supporting a light-sensitive system and using a silver compound to obtain permanent metallic contrasts. For instance, it has been proposed to produce photographic contrasts by means of a sensitized layer containing a mixture of a photosensitive diazocompound and a silver salt which layer after exposure was treated with an alkaline liquid and then with a fixing agent to obtain a silver picture.

The object of this invention is to produce silver pictures having better contrast effects than the above-mentioned.

According to the invention a light-sensitive system which is converted upon exposure to light into one or more substances referred to hereinafter as the light-reaction product is reacted with a silver compound to reduce the latter and form a silver germ image under carefully con trolled conditions. The silver germ image is then subsequently intensified by physical development.

In particular the method according to the invention produces improved photographic contrasts by carefully controlling the conditions leading to the formation of the contrast in a phase intermediate the light-reaction phase and the phase of intensification of the silver-germ image, namely a phase of silver-germ formation. In this phase a suitable medium is used to increase the difference in potential between the silver potential and the normal light redox potential of the light-reaction product after exposure of the light-sensitive material. The sensitivity of the system is thereby improved to such an extent that at least one-third less light is required to form the silver-germ image than that required in the absence of an increase in the difference in potential. The silver compound necessary to produce said silver-germ image may be introduced into a carrier for the light-sensitive material before exposure to light or the silver compound may be introduced in the carrier for reaction with the light-reaction product after exposure.

The term ,silver-germ image the product obtained by the photochemical conversion of light-sensitive substances.

The term silver-germ image is herein dfiened as a silver nuclei image produced by the action of the lightreaction product upon a silver compound reducing the latter to metallic silver. The silver-germ image corresponds to the latent image in a silver halide system and is weak and invisible to the eye.

The term light redox potential is herein defined as the potential assumed by a platinum electrode measured with respect to a normal hydrogen electrode in a solution of the light-reaction product and of the product obtained therefrom upon oxidation thereof by silver ions. The normal light redox potentia is further defined as the potential measured between a platinum electrode and a normal hydrogen electrode in the above solution in which the molecular concentrations of the light-reaction product and the product obtained therefrom upon oxidation by silver ions are equal to unity. In practice it is suflicient that these concentrations are equal.

The term silver potential is herein defined as the potential assumed by a silver electrode in a solution containing silver ions measured with respect to a normal hydrogen electrode.

The term physical development is herein defined as a development of the contrast in which the metal required for contrast formation is supplied from a liquid containing ions of the metal required for contrast formation and a reducing agent.

It should be noted that the present invention is not applicable to silver halide systems.

In order toobtain better contrasts it has been found that the difference in potential AE between the silver pois herein defined as a tential and the normal light redox potential must be increased, i.e., the silver potential must be increased and/or the normal light redox potential must be increased during the formation of the silver-germ image. However, not any increase in AB leads to an increase in dispersions of silver in the silver-germ image. It has been found that no appreciable effect can be found in the photographic sensitivity of the system if AB is increased to a value smaller than 0.2 volt during the formation of the silver-germ image. Furthermore, it has been found that an increase in AB to a value greater than 0.8 volt no longer results in a greater intensification of the silver-germ image.

Exact ascertainment of the reducing power AB prevailing during formation of the silver-germ image is not very feasible, since it Would require potential measurements in the layer, which are unsuitable in practice. However, as will be seen hereinafter, such measurements are not essential for properly carrying out the methods according to the invention.

In order to ascertain the AE prior to taking the steps according to the invention the value of the silver potential is determined in a solution in which the molecular concentration of the silver compound is equal to that in the solution by means of which the layer is provided with silver compounds. Then the value of the normal light redox potential is determined in a solution in which the essential conditions are the same as those prevailing in the sensitising solution by means of which the layer is provided with thte light-sensitive substance. Consequently, the pH of the measuring solution should be given the same value as the pH of the sensitising solution, whilst substances combining more strongly with the oxidation product of the light-reaction product than with the light-reaction product itself, if they are present in the sensitising solution, must be added in the same molecular concentration to the measuring solution. Obviously, in performing the measurements, allowance must be made for further treatments of the layer (which, as the case may be, are effected prior to exposure), always provided that said treatments result in an increase of the normal light redox potential.

It has been found that the intensifiability of a silvergerm image is dependent upon its degree of dispersion, that is, the number of germs into which a certain amount of germ silver is divided. It therefore appears that an increase in the dilference in potential between the silver potential and the normal light redox potential (AE) results in an increase of the number of germs per unit sur- =HoWever, no general relationship between the increase in AB and the increased number of germs can be specified since the relationship is a particular one for each particular system. The relationship is however, applicable for increases in AB within the limits set forth above.

The most important method of decreasing the normal light redox potential consists in maintaining the photosensitive layer acidic during exposure and increasing the pH of the exposed layer by means of a suitable medium, for. example an alkaline vapour or liquid, or a buffer solution, the pH of which is higher than the pH of the la er.

Intensification of the silver-germ image is accomplished by means of a physical developer containing silver ions, a reducing agent and substances such as acids, alkalies and complexing salts to regulate the rate of development or enhance stability of the solution. Mercury and platinum ions may be substituted for silver ions in the developing solution. Generally acids are used to stabilize the solution and control the rate of development although alkaline substances and alkali metal sulphites may be used successfully. If citric acid is used for this stabilizing purpose the contrast becomes a neutral grey color shortly after the beginning of development resulting in contrasts of satisfactory color having a lower gradation than in contrasts obtained with the use of other developers. The development factor increases as the time of development is longer.

The intensification of the silver-germ image by the method according to the invention is so high that the required amount of light required to form silver-germ images by the method according to the invention is up to 1000 times less than that used in methods hitherto known.

If silver compounds spontaneously capable of providing germs and insoluble silver compounds are deposited at unwanted places they will also be intensified by physical development thereby neutralizing the contrast in the enveloping fog which frequently may have a blackening even higher than 2. a This fogging may be eliminated by treating the silvergerm image with a liquid capable of dissolving metallic silver and/or silver compounds. For this purpose treatment with a dilute oxidizing acid, such as nitric acid, or a solution containing ferric cyanide and thiosulphate is particularly suitable. These liquids must only react for a short time and their concentrations must not be unduly high otherwise the silver-germ image may also be dissolved.

In cases where the pH of the medium during the formation of the silver-germ image is at least temporarily higher than 11, it may be necessary to dissolve metallic silver. However, these baths are preferably limited to those cases in which their use is unavoidable.

If the pH value during the formation of silver germs is not unduly high a solvent may be utilized to dissolve only the silver compounds. Such is generally the case if the pH value remains below 9 and paper serves as the support for the light-sensitive material. If regenerated cellulose is used a pH value up to 11 is permissible. By means of this method substantially no silver germs are dissolved during the treatment and only a slight loss in sensitivity occurs. For the removal of silver compounds soluble salts of thiosulphuric acid, thiocyanic acid or sulphuric acid and preferably the alkali metal or ammonium salt may be used.

The concentration of the bath and the reaction period should be adjusted to avoid the dissolution of the silvergerm image. For example, a 5% aqueous solution of sodium thiosulphate is used to eliminate the fogging of silver compounds by washing for 3 minutes without a resulting loss of sensitivity.

Fogging always occurs if the pH of one of the treatments is greater than 7 to 7.5.

v For ease of treatment, a layer which has been exposed and contains an amount of silver compounds sufficient to form a silver-germ'image may be treated with an alkaline liquid which can dissolve silver compounds to avoid fogging while simultaneously increasing the pH of the system. A solution'of thiosulphate which has its pH adjusted to a value between 8 and 12 by the addition of buffering substances and/or alkalies is particularly suitable for this purpose. A solution of ammonium hydroxide may similarly be used.

The silver compounds may be conveniently added to the sensitising solution used to introduce the light-sensitive compound into the layer before exposure.

It is possible to obtain metallic images having wide limits of gradation, such as values from 0.5 to 10, by a suitable choice of the silver ion concentration and the pH value during the formation of the silver germ image. As the silver ion concentration of the bath is increased the gradation obtained is higher. In addition the pH value during formation of the silver germ image also appears to have an effect upon the gradation of the contrast.

Among the organic substances particularly suitable are aromatic quinones and their derivatives which upon irradiation change to polyvalent phenols, for example ben'zoquinone and anthraquinone-sulphonic acids.

An important use of the described method is that in which a color contrast in situ with a metal contrast is obtained with the use of physical color developers more particularly developers containing color couplers as taught by copending U.S. patent application Serial No. 227,564, filed May 22, 1951, and now US. Patent 2,750,292.

The invention will now be described in connection with the following examples.

7 Example I A foil of superficially saponified cellulose acetate is sensitised by impregnation for 2 minutes in 0.1 n nitric acid in which 0.75% of benzoquinone and 0.5 of silver nitrate are dissolved, and subsequently dried.

Exposure behind a sensitometer with the use of a mercury-vapour lamp is followed by treatment with a 0.5 n solution of ammonia in water for 2 minutes. After washing, development takes place for 2 minutes in a de-' veloper consisting of 1% of metol, 3.5% of citric acid and 0.4% of silver nitrate in water. A satisfactory, neutral-grey copy of weak gradation is obtained. The gain in sensitivity as compared with the cases in which the ammonia treatment and the physical development respectively are omitted, is certainly a factor 25 to 5 0. The reducing power AB is about 0.4 volt at pH=7.

In this example and in the other example use may alternatively be made of physical color development, for example, a blue green color may be produced using a developer containing 0.08% of dimethylamine-4-phen yl aminenitrate, 5% of tartaric acid and'0.4% of silver nitrate, to which 2 ccs. of a solution of 10% of naphthol-l in ethanol is added per 25 ccs. The silver image may be removed with 'a liquid containing potassium ferricyanide and thiosulphate.

Example 11 A foil of superficially saponified cellulose acetate is sensitised by impregnation in 0.1 n nitric acid in which 2% of anthraquinoneQJ-disulphonic acid sodium salt is dis solved, and subsequently dried. The pH of the sensitising solution is 1.1. Exposure behind a sensitometer with the use of a mercury-vapour lamp is followed by treatment for 1 minute.

(a) With a solution of 0.15 n silver nitrate and 0.1 n nitric acid. 1

The pH of solution (a) is 1.1.

(b) With a solution of 0.15 11 silver nitrate and 0.5 11

sodium lactate.

The pH of solution (11) is 7.3.

After washing for 3 minutes in Water, physical development takes place for 4 minutes in a developer consisting of 1% of metol, 3.5% of citric acid and 0.4% of silver nitrate.

'In case (a) AE at pH=-l.l is 0.6 volt and is'not increased during the formation of the silver-germ image.

In case (b) AE during the formation of the silvengerm image is increased to a value greater than 0.8 volt by increasing the pH of the layer.

The gain in sensitivity in case (b) as compared with case (a) is certainly a factor 50.

While the invention has thus been described in connection with specific examples and applications thereof, other modifications and substitutions will be obvious to those skilled in the art, without departing from the spirit and the scope of the invention as defined in the appendant claims.

What is claimed is:

1. A method of producing photographic images com prising the steps of selectively exposing a layer having a pH of less than about 7.0 and containing a light-sensitive compound selected from the group consisting of light-sensitive benzoquinones, benzoquinonesulfonic acids and anthroquinone sulfonic acids, to decompose said compound at selected portions of the layer, said layer after exposure having a AB in the presence of a water soluble silver salt of less than 0.8 volt, treating the exposed layer with an aqueous alkaline solution in the presence of said water soluble silver salt, thereby increasing the pH of the layer and thereby forming a latent silver image, the alkalinity of said aqueous alkaline solution being sufiicient to increase the AE to a value greater than at least 0.2 volt, and physically developing the la tent image by then applying to said layer a developing solution containing a salt of a metal higher than copper in the electromotive series, said metal having a single electrode potential the negative value of which is greater than 0.47 volt, said salt being reducible to metal in solution and a water soluble organic reducing agent.

2. A method of producing photographic images comprising the steps, selectively exposing a layer having a pH of less than about 7.0 and containing a light-sensitive benzoquinone, to decompose said benzoquinone at selected portions of the layer, said layer after exposure having a AB in the presence of a Water soluble silver salt of less than 0.8 volt, treating the exposed layer with an aqueous alkaline solution in the presence of said water soluble silver salt, thereby increasing the pH of the layer and thereby forming a latent silver image, the alkalinity of said aqueous alkaline solution being sufiicient to increase the AE to a value greater than at least 0.2 volt, and physically developing the latent image by then applying to said layer a developing solution containing a salt of a metal higher than copper in the electromotive series said metal having a single electrode potential the negative value of which is greater than 0.47 volt, said salt being reducible to metal in solution and a Water soluble organic reducing agent.

3. A method of producing photographic images comprising the steps, selectively exposing a layer having a pH of less than about 7.0 and containing a light-sensitive anthroquinone sulphonic acid to decompose said anthroquinone compound at selected portions of the layer, said layer after exposure having a AB in the presence of a water soluble silver salt of less than 0.8 volt, treating the exposed layer with an aqueous alkaline solution in the presence of said Water soluble silver salt, thereby increasing the pH of the layer and thereby forming a latent silver image, the alkalinity of said aqueous alkaline solution being suflicient to increase the AE to a value greater than at least 0.2 volt, and physically developing the latent contrast in the image by then applying to said layer -a developing solution containing a salt of a metal higher than copper in the electromotive series, said metal having a single electrode potential the negative value of which is greater than 0.47 volt, said salt being reducible to metal in solution, and a water soluble organic reducing agent.

4. The method of claim 1 in which the silver salt is added to the light-sensitive layer before exposure of the layer.

5. The method of claim 1 in Which the silver salt, together with the aqueous alkaline solution, is added to the light-sensitive layer after exposure of the layer.

6. The method of claim 1 in Which the reducible salt in the developing solution is selected from the group consisting of silver and mercury salts and a color coupler is present in said solution.

7. The method of claim 1, in which the aqueous alkaline solution is a solution of ammonia.

8. The method of claim 1, in which the aqueous alkaline solution is an aqueous alkaline solution of an alkali metal thiocyanate.

9. The method of claim 1, in which insoluble silver containing materials formed in unexposed areas are dissolved before subjecting the system to physical developmg.

10. A method of producing photographic images comprising the steps, selectively exposing a layer having a pH less than about 7.0 and containing a light-sensitive compound selected from the group consisting of lightsensitive benzoquinones, benzoquinonesulfonic acids and anthnoquinone sulfonic acids, to decompose said compound at selected portions of the layer, said layer after exposure having a AB in the presence of a water soluble silver salt of less than 0.8 volt, treating the exposed layer with a solution of an alkali metal thiosulphate in the presence of said water soluble silver salt, thereby increasing the pH of the layer and thereby forming a latent silver image, the alkalinity of said thiosulphate solution being sufficient to increase the AE to a value greater than at least 0.2 volt, and physically developing the latent image by then applying to said layer a developing solution containing a salt of a metal higher than copper in the electromotive series, said metal having a single electrode potential the negative value of Which is greater than 0.47 volt, said salt being reducible to metal in solution and a water soluble organic reducing agent.

11. The method of of the thiosu'lphate is 8 to 12.

claim 10, in which the solution adjusted to a pH value of about References Cited in the file of this patent UNITED STATES PATENTS 1,843,822 Kogel Feb. 2, 1932 2,066,918 Poser et a1. Ian. 5, 1937 FOREIGN PATENTS 322,430 Great Britain Dec. 2, 1929 

1. A METHOD OF PRODUCING PHOTOGRAPHIC IMAGES COMPRISING THE STEPS OF SELECTIVELY EXPOSING A LAYER HAVING A PH OF LESS THAN ABOUT 7.0 AND CONTAINING A LIGHT-SENSITIVE COMPOUND SELECTED FROM THE GROUP CONSISTING OF LIGHT-SENSITIVE BENZOQUINONES,BENZOQUINONESULFONIC ACIDS AND ANTHROQUINONE SULFONIC ACIDS, TO DECOMPOSE SAID COMPOUND AT SELECTED PORTIONS OF THE LAYER, SAID LAYER AFTER EXPOSURE HAVING A $E IN THE PRESENCE OF A WATER SOLUBLE SILVER SALT OF LESS THAN 0.8 VOLT, TREATING THE EXPOSED LAYER WITH AN AQUEOUS ALKALINE SOLUTION IN THE PRESENCE OF SAID WATER SOLUBLE SILVER SALT, THEREBY INCREASING THE PH OF THE LAYER AND THEREBY FORMING A LATENT SILVER IMAGE, THE ALKALINITY OF SAID AQUEOUS ALKALINE SOLUTION BEING SUFFICIENT TO INCREASE THERE TO A VALUE GREATER THAN AT LEAST 0.2 VOLT, AND PHYSICALLY DEVELOPING THE LATENT IMAGE BY THEN APPLYING TO SAID LAYER A DEVELOPING SOLUTION CONTAINING A SALT OF A METAL HIGHER THAN COPPER IN THE ELECTROMOTIVE SERIES, SAID METAL HAVING A SINGLE ELECTRODE POTENTIAL THE NEGATIVE VALUE OF WHICH IS GREATER THAN 0.47 VOLT, SAID SALT BEING REDUCIBLE TO METAL IN SOLUTION AND A WATER SOLUBLE ORGANIC REDUCING AGENT. 